In filtration systems which have repeated or cyclical fluctuations in the flow rate across a filter, fatigue failure of the filter can be a problem. This is especially true for systems with wide flow excursions, e.g., from zero to full flow and back to zero, such as are experienced in filtration systems for piston-type pumps as used in fluid power systems. The flow cycles cause coincident cycles in the differential pressure across the filter, typically resulting in a "breathing type" flexure within the pleated medium of the filter. If the pleated medium is a composite having a filter layer and a support and drainage layer, as the pleated medium flexes, the support and drainage layer can rub back and forth along the filter layer. Because the support and drainage layer is typically much more coarse than the filter layer, this rubbing may produce weak spots which can rupture in service. This type of failure is known as a fatigue failure.
Another problem of conventional filters is that they may have a low dirt capacity when the liquid flows through the filter element unevenly. For example, more liquid may flow through the upper portion of a filter element than through the lower portion. Consequently, more dirt is deposited on the upper portion of the filter element than on the lower portion. This uneven loading of the filter element can lower the dirt capacity of the filter element. The lower the dirt capacity of a filter element, the more often it is necessary to replace the filter, resulting in increased material and labor costs.